Conventionally, in an image forming apparatus, for example, a DC power source is incorporated for outputting a direct current in two systems of 5 VDC and 24 VDC. Normally, 5 VDC is supplied to a control-type logic portion, while 24 VDC is supplied to a drive system high-current consumption portion such as a motor. Some of the image forming apparatuses are provided with a so-called interlock mechanism (interlock switch) for mechanically shutting off a 24 VDC power source of a motor, a high-voltage circuit and the like so as to prevent a user from being injured when the user opens an exterior cover of the image forming apparatus in order to deal with troubles such as paper jam. Thereby, when the exterior cover is opened, for example, an interlock switch is opened in conjunction therewith, and power supply to a load that is connected to a subsequent stage side of the interlock switch is interrupted.
FIG. 5 is a diagram showing a main part of an image forming apparatus provided with a conventional interlock mechanism. In the diagram, 101 denotes a 24 VDC power source (simply referred to as DC power source); 102 denotes an interlock switch; 103 denotes a motor; 104 denotes a high-voltage substrate; 105 denotes a photoreceptor drum; 106 denotes a developing roller; and 107 denotes a charger. Further, the high-voltage substrate 104 includes a diode 104a, a capacitor 104b, a developing positive bias circuit 104c, and a developing reverse bias circuit 104d. 
FIG. 6A to FIG. 6C are diagrams for explaining a status of developing bias voltage at the time of starting printing in the image forming apparatus of FIG. 5. The high-voltage substrate 104 is configured by connecting the developing positive bias circuit 104c to the developing reverse bias circuit 104d in parallel. The developing positive bias circuit 104c generates predetermined developing positive bias voltage, and the developing reverse bias circuit 104d generates predetermined reverse bias voltage. Additionally, the high-voltage substrate 104 generates developing bias voltage by combining developing positive bias voltage with developing reverse bias voltage to apply the generated developing bias voltage to the developing roller 106.
For example, as shown in FIG. 6A, in the case of requiring “−450 V” as developing bias voltage, in the high-voltage substrate 104, “−550 V” in the developing positive bias circuit 104c and “+100 V” in the developing reverse bias circuit 104d are generated, those of which are combined to generate “−450 V” as the developing bias voltage. Here, in FIG. 6B, at the time of stoppage, the surface of the photoreceptor drum 105 is not charged, surface potential of which becomes “0 V”. Then, at the time of starting printing, the photoreceptor drum 105, the charger 107 and the developing roller 106 are electrified, and when the photoreceptor drum 105 starts to rotate in an arrow direction, a left part of an X point of the photoreceptor drum 105 becomes charged to “−640 V” by the charger 107, however, charging is not applied between the X point and a Y point, and surface potential of an X-Y part thus remains in “0 V”. Therefore, to the X-Y part passing through the developing roller 106, a toner with negative polarity is forcibly attached.
On the other hand, as shown in FIG. 6C, a pre-rotation processing is performed before starting printing to prevent a toner from being attached to the X-Y part. That is, while the X-Y part of the photoreceptor drum 105 passes through the developing roller 106 from the start of rotation of the photoreceptor drum 105, developing reverse bias voltage of “+100 V” is applied to the developing roller 106 as developing bias voltage (development actual high-voltage output). Thereby, the toner with negative polarity is attracted to the developing roller 106 side for preventing a toner from being attached to the X-Y part of the photoreceptor drum 105. This is a technique which is generally performed in a color copier for which two-component developer (toner or carrier) is mainly used.
Here, two-component developer is composed of a toner having a non-magnetic body and a carrier having a magnetic body. A main component of the carrier is iron, and held on the developing roller 106 by magnetic force of the developing roller 106 as a magnet roller, electrically having polarity (+) opposite to that of developing bias voltage. Thus, there is no problem in a case where surface potential of the photoreceptor drum 105 is −640 V and developing bias voltage is −450 V, however, when a potential difference thereof becomes large, electric force becomes greater than magnetic force, so that a carrier is attached onto the photoreceptor drum 105, which poses a problem.
For example, FIG. 6B assumingly shows a case where emergency stop is performed during printing, and the interlock switch 102 (FIG. 5) is turned off. At the time, in the photoreceptor drum 105, the X point is assumed to be in a position of the charger 107, and the Y point is assumed to be in a position of the developing roller 106. In this case, the X-Y part of the photoreceptor drum 105 is charged to “−640 V” by the charger 107. Then, the interlock switch 102 is turned off, whereby the developing roller 106 is stopped, so that developing bias voltage becomes 0 V. On the other hand, the photoreceptor drum 105 rotates through inertia even after emergency stop, and the X-Y part of the photoreceptor drum 105 passes through the developing roller 106. At the time, developing bias voltage of the developing roller 106 is “0 V”, and surface potential of the X-Y part of the photoreceptor drum 105 is “−640 V”. Therefore, a potential difference becomes large, and electric force becomes greater than magnetic force, so that a carrier is attached onto the photoreceptor drum 105.
On the other hand, for example, Japanese Laid-Open Patent Publication No. 2002-196549 describes a technique for changing in a phased manner developing bias voltage in order to prevent a carrier from moving from a developing sleeve to a photoreceptor side at the time of emergency stop due to paper jam during image forming operation, opening of a door by a user, and the like.
As shown in FIG. 5 described above, the capacitor 104b is provided in the high-voltage substrate 104 so as to be able to output “−450 V” as developing bias voltage by electric power accumulated in the capacitor 104b in which the photoreceptor drum 105 rotates through inertia after the interlock switch 102 is turned off due to emergency stop for a given length of time. Thereby, a potential difference between developing bias voltage of the developing roller 106 and surface potential of the X-Y part of the photoreceptor drum 105 is made smaller, so that a carrier is prevented from being attached to the drum while the drum rotates through inertia.
However, in FIG. 5, in a case where a plurality of loads such as the high-voltage substrate 104 and the motor 103 are connected to the DC power source 101, a counter electromotive current is generated by inertial rotation of the motor 103 at the time of emergency stop, which comes around the inside of the high-voltage substrate 104 in some cases. This case results in continuous driving of the developing reverse bias circuit 104d which is a circuit part unnecessary for the high-voltage substrate 104 by the counter electromotive current, so that charged voltage of the capacitor 104b is consumed in both the developing positive bias circuit 104c and the developing reverse bias circuit 104d. Thus, it is difficult to sufficiently secure an output holding time of developing bias voltage (developer holding time) so that a capacitor having large capacity is needed for sufficiently securing the developer holding time.
The image forming apparatus described in the above-described. Japanese Laid-Open Patent Publication. No. 2002-196549 is configured to change developing bias voltage in a phased manner in order to prevent a carrier from moving from a developing roller to a photoreceptor drum side, however, not intended to disclose a developing positive bias circuit and a developing reverse bias circuit, nor to solve the above-described problem.